1. Field of the Invention
The present invention relates to a charging device, an image forming apparatus, and a charging unit.
2. Description of the Related Art
Known as an electrophotographic image forming apparatus is one which uses a photosensitive drum as an image carrier to charge its surface, exposes the surface to light, and accordingly forms an electrostatic latent image, attaches toner to the electrostatic latent image to make the image visible, and transfers the image onto paper to form an image.
In such an image forming apparatus, a corotron and a scorotron charging device that use a corona discharge is in practical use as a charging device that charges a photosensitive drum. In these devices, a corona discharge member/discharge electrode functions, for example, to generate a corona discharge and charge the surface of the photosensitive drum by ions produced by the discharge by tightly stretching (meaning providing in a stretched state) a discharge wire with a small diameter and applying a high voltage thereto. When a positive or negative high voltage is applied to the discharge wire, fine particles floating in the air, which have been reversely charged, for example, foreign objects such as toner particles and paper powder are attracted and attached to the discharge wire. Hence, there is a problem that if a high voltage is continued to be applied to the discharge wire, the discharge wire becomes dirty due to the accumulation of the attached foreign objects, which leads to that a corona discharge does not occur normally.
In order to deal with the above problem, some image forming apparatuses including a constant-current power supply to apply a voltage to a corona discharge wire, a unit to detect the applied voltage to the corona discharge wire, and a cleaning control unit have also a mechanism for cleaning the corona discharge wire. In such image forming apparatuses, the surface of the wire becomes damaged over time due to the influence of the cleaning, which results in the shortening of the life of the wire.
In order to deal with the above problem, the following technologies are known (for example, see Japanese Laid-open Patent Publication No. 61-015163 and Japanese Laid-open Patent Publication No. 61-084665). In other words, a known electrophotographic image forming apparatus has a feature that an airflow to a charging device generates an air curtain from the flow of an airflow flowing from an upper opening portion of a shield plate to a discharge wire, a flow on an exhaust side is inhibited, and foreign object contamination into the charging device is prevented. Moreover, there is a feature that only a flow velocity in the vicinity of an insulation block supporting the discharge wire is higher than the other places, as a feature of the flow of an airflow.
Moreover, apart from the above, it is configured to prevent foreign object contamination of the discharge wire only with a configuration to flow an airflow from the upper side of a charger (for example, see Japanese Laid-open Patent Publication No. 7-134532). In this technology, the flow of an airflow by a difference in flow velocity and the area and position to which air is flown are not specified.
Therefore, there is a need to provide a charging device having a configuration with an airflow effect where foreign objects are not attracted and attached to a discharge wire by making good use of an airflow to deal with reduction in the life of the discharge wire due to foreign object contamination.